While Mus81 knockout mice, from which the selectable marker was removed, were born at expected Mendelian frequencies and were indistinguishable from wild type littermates in terms of development, growth, immune function and fertility [17], our Fibulin-4 hypomorphic mice displayed a 2-fold lower expression of Fibulin-4 in heterozygous Fibulin-4+/R aortas and a 4-fold downregulation in homozygous Fibulin-4R/R aortas, resulting in ECM defects and vascular abnormalities, including aortic aneurysms in the aorta ascendens [14]. The gene discussed is MUS81; the disease is aortic aneurysm.